What is formed when formaldehyde (methanal) is heated with concentrated sodium hydroxide solution?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Sodium formate and methanol
Formaldehyde (HCHO) has no α-hydrogen, so with concentrated alkali it undergoes the Cannizzaro reaction, a disproportionation: one molecule is reduced to methanol and the other is oxidised to the salt of methanoic acid, sodium formate.
\( \displaystyle \ce{2HCHO + NaOH} \xrightarrow[\Delta]{\text{conc.}} \ce{CH3OH + HCOONa} \)
Why the other options are incorrect
Option (2) Sodium acetate and methanol — Incorrect. Acetate has two carbon atoms; the oxidised half of a one-carbon aldehyde can only be the one-carbon formate.
Option (3) 2-Hydroxyethanal — Incorrect. Joining two HCHO molecules in an aldol-type reaction needs an α-hydrogen, which formaldehyde does not have.
Option (4) Methanol and carbon dioxide — Incorrect. The oxidised molecule ends up as a carboxylic acid salt (sodium formate), not as carbon dioxide.
NCERT (2026-27 edition):
8.4 Chemical Reactions – “(i) Cannizzaro reaction: Aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.”